Most thermostats automatically turn on the heat when the room or zone temperature drops below the heating set point and then start the air conditioning when the room or zone temperature exceeds the cooling set point.
During the early morning hours if the room or zone temperature drops below the heating set point heat will be turned on regardless of current or upcoming outdoor weather conditions. Likewise in the late afternoon if the room or zone temperature exceeds the cooling set point the thermostat starts the air conditioning regardless of current or upcoming outdoor weather conditions. This could result in heating in the morning and then air conditioning a couple hours later or air conditioning in the late afternoon and heating a couple hours later.
Similar shortcomings exist with conventional Packaged Terminal Air Conditioners. A Packaged Terminal Air Conditioner (abbreviated as PTAC) is a type of self-contained heating and air conditioning system commonly used in hospitals, hotels, motels, senior housing facilities, condominiums, apartment buildings, schools and student dormitories. Most PTACs are used to heat or cool a single room using only electric heaters and or heat pumps for heating. Some PTACs are cooling only. Other PTACs are equipped with hydronic heating coils or natural gas heating.
Typically the PTAC is connected to a wall thermostat with switches. The switches control the heating or cooling mode of operation and the fan settings. During warm weather someone in the room must switch the thermostat to cooling mode for the cooling to work. Once the cooling mode is selected the cooling will be automatically controlled by the thermostat temperature set point. When the outside temperature gets cold someone in the room must switch the thermostat to heating mode and the heating will be automatically controlled by the thermostat temperature set point. If the days are hot and the nights are cool then someone will needs to switch the thermostat to heating at night and back to cooling as the room gets hot during the day. This constant switching of the thermostat results in damage to the thermostat and wasted energy by manually switching between cooling and heating.